recursive pattern

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi! I am trying to match C++ templates:

#!/usr/bin/perl

$cparens = qr/
(?<intern>
 \<
 (?:
  [^<>]++
  |
  (?$+{intern})
 )*+
 \>
)
/x;

$ks="BinaryNode<OpAdd1<OpAdd2>,OpMulAdd2<OpMulAdd>>";

if ($ks =~ m/
([^<>]++)
    \<(?'part1'[^<>]++(?:$cparens)?)
    \,(?'part2'[^<>]++(?:$cparens)?)
    \>
    /x) {
  print "ok\n";
}

print "p1 $+{part1}\n";
print "p2 $+{part2}\n";
[download]

This matches as supposed and prints the part1 and part2 of the toplevel template:

p1 OpAdd1<OpAdd2>

p2 OpMulAdd2<OpMulAdd>

But, when I add one more level of recursion, like adding to one of the parts:

$ks="BinaryNode<OpAdd1<OpAdd2<a>>,OpMulAdd2<OpMulAdd>>";
[download]

it doesn't match anymore!

I don't follow this logic. Does anybody know why this behaviour is shown?

Frank

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Re: recursive pattern by ikegami (Patriarch) on Jan 05, 2010 at 22:28 UTC
What's with people not checking for errors and warnings when their code doesn't work? `Use of uninitialized value $+{"intern"} in regexp compilation at a.pl +line 6.` [download] It actually fails when any recursion is needed. The initial expression doesn't actually exercise the recursive path, which is why it works. The problem results in `$cparens` containing `(?x-ism: (?<intern> \< (?: [^<>]++ \| (?) )*+ \> ) )` [download] Replace `(?$+{intern})` [download] with `(?&intern)` [download]	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re: recursive pattern
by ikegami (Patriarch) on Jan 05, 2010 at 22:28 UTC

What's with people not checking for errors and warnings when their code doesn't work?

Use of uninitialized value $+{"intern"} in regexp compilation at a.pl 
+line 6.
[download]

It actually fails when any recursion is needed. The initial expression doesn't actually exercise the recursive path, which is why it works.

The problem results in $cparens containing

(?x-ism:
(?<intern>
 \<
 (?:
  [^<>]++
  |
  (?)
 )*+
 \>
)
)
[download]

(?$+{intern})
[download]

(?&intern)
[download]

[reply]
[d/l]
[select]